you talk about the problem with extra fuel needed yet mention that there are some trying to develop a reusable second stage which incidently will require fuel

so lets watch the youtube video about why single stage rockets suck right? oh shit conbustible

meanwhile i'm talking about different technologies

hypersonic and sabre engines take advantage of the air in the atmosphere so they don't need to carry so much oxidizer with them

on a very quick search designs similar to this

https://physicsworld.com/a/air-breathing-rocket-engines-the-future-of-space-flight/

or this

[https://www.researchgate.net/publication/27524937_StarRunner_A_Single-Stage-to-Orbit_Airbreathing_Hypersonic_Propulsion_System]

still looking further ahead there are other technologies being explored and imho that can be explored further